Anaerobic formulations, i.e. (meth)acrylic ester formulations which are oxygen stabilized and cure as a result of oxygen deprivation, have become well known since their disclosure in U.S. Pat. No. 2,628,178. The invention of formulations employing hydroperoxide catalysts which are stable without aeration which was disclosed in U.S. Pat. No. 2,895,950 (Krieble) prompted a wide range of commercial uses for such formulations including thread locking, impregnation of porous articles, gasketing and adhesive bonding applications. Acceleration of the cure of such composition has been disclosed in numerous patents. Sulfimide accelerators, particularly saccharin (benzoic sulfimide), have been used in most anaerobic formulations since the disclosure of their utility in U.S. Pat. No. 3,046,262. Other accelerator systems which are known for use in anaerobic formulations include various amine compounds as disclosed, for instance, in U.S. Pat. Nos. 3,041,322, 3,203,941 and 3,218,305 and various compounds having structures analogous to sulfimides such as disclosed in U.S. Pat. No. 4,513,127 and U.S. Pat. No. 4,622,348 and the references disclosed therein. Certain acyl hydrazine compounds are disclosed as anaerobic accelerators in U.S. Pat. Nos. 4,180,640 and 4,287,330, preferably in combination with an acid having a small pKa of 6 or less. The various known types of accelerators are often used in combination in commercial formulations.
Similar acrylic curable peroxy initiated compositions which rely on chemical activation rather than oxygen deprivation to activate cure initiation are disclosed, for instance, in U.S. Pat. Nos. 4,442,267; 4,450,030; 4,451,615 and 4,574,142. Preferred activators for such compositions are amine/aldehyde condensation products.
Formulations employing combinations of anaerobic and chemical activation are also known. For instance, in U.S. Pat. No. 3,672,942 there is described an impregnation process employing the steps of impregnating a porous article with an anaerobically curable composition and then treating the surface with a chemical activator to induce surface cure where the impregnant is exposed to oxygen. Suitable activators include solutions of amine/aldehyde condensation products, various metal salts or chelates and various thiourea or other sulfur containing compounds. While the presence of uncured surface monomer has been reduced by the improved processes of U.S. Pat. No. 3,969,552 and U.S. Pat. No. 4,165,400, it is still generally advised to use an accelerator rinse to activate cure of the composition nearest of the surface of the impregnated article. Erythrobic or ascorbic acids have become preferred impregnation surface activators in more recent years.
It has also long been known that transition metals, especially copper or iron, play an important part in the cure mechanism of peroxide initiated acrylic formulations. This is especially so in anaerobic cure mechanisms. Transition metal oxidation is often part of the redox reaction which produces free radicals from peroxy initiators. Transition metal may be supplied by the substrate itself or in the form of salts of metals incorporated into the formulation or applied to the substrate. Copper, manganese, iron, cobalt and vanadium salts are all known for this purpose, copper and iron being the most commonly used. In impregnation processes which employ aeration of the acrylic monomer formulation it is typical to add 1-10 ppm copper as a copper salt directly to the formulation. As disclosed in U.S. Pat. No. 4,632,945, combinations of copper and iron salts or complexes may also be employed.
The participation of xanthate and dithiocarbamate compounds in free radical reactions has been previously reported. The participation of dithiocarbamate and xanthate salts in free radical reactions appears very complex and occasionally contradictory. In rubbers which include unsaturated sites (e.g. natural rubber, polybutadienes and various unsaturated copolymers), these compounds are frequently used as vulcanizing agents, usually in conjunction with at least one other compound. Examples are the abstracts of JP (1982)/102974 and EP 184301. Similar activity is reported for saturated rubbers containing small amounts of epoxy or halo groups in U.S. Pat. Nos. 3,525,721 and 4,524,185. Fe or Mg dialkyldithiocarbamates cause cure of mercaptan terminated polymers in the presence of air in U.S. Pat. No. 3,991,039).
Use of xanthate or dithiocarbamate salts is taught as part of a peroxy catalyst system for emulsion polymerization of chloroprene monomer and comonomer mixtures in U.S. Pat. No. 4,814,401.
In U.S. Pat. No. 3,639,370 dialkyldithiocarbamates with an unsaturated peroxy compound such as t-butyl permalinate or t-butyl perethyl fumerate are taught as a low temperature polymerization initiation system for vinyl compounds such as acrylic esters and unsaturated polyesters.
On the other hand, dithiocarbamates are also reported as free radical inhibitors in other polymerization or vulcanization reactions.
In U.S. Pat. Nos. 3,378,510 and 4,632,950 dialkyldithiocarbamate salts are disclosed as scorch retarders (on i.e. inhibitors of premature vulcanization) for peroxy vulcanized saturated rubbery polymers. (U.S. Pat. No. 4,632,950 also identifies chlorinated polyethylene, 1,4-polybutadiene and fluoroelastomers as suitable polymers.)
U.S. Pat. No. 4,018,604 teaches dialkyldithiocarbamates (e.g. Zn dimethlydithiocarbamate) as an additive to prevent fogging of non-silver free radical photosensitive films.
U.S. Pat. No. 4,021,310 teaches use of O.sub.2, copper dialkyldithiocarbamates and a conventional inhibitor such as hydroquinone as an inhibition system against polymerization during fractional distillations of acrylic monomers. Table 1 of this reference demonstrates that the dithiocarbamate is an essential ingredient of this inhibitor system.
JP (1986)/76578 describes a two-liquid type resin composition utilizing an acrylic monomer. A peroxide (e.g. cumene hydroperoxide) is placed in one part of the composition and a vanadium compound, together with a nickel xanthate compound are used in the second part. At least a portion of the monomer must have an additional functional group, as for instance a carboxylic or phosphate acid group, a hydroxy, amino amide or ether group. This reference discloses that the xanthate compound acts as a stabilizer against decomposition of the vanadium compound, and as an inhibitor against premature gelling of the vanadium containing monomer solution, not as an active ingredient of the cure system.